For cosmetic purposes, contact lenses having one or more colorants dispersed in the lens or printed on the lens are in high demand. These colored contact lenses enhance the natural beauty of the eye, or provide unique patterns on the iris of the wearer, or provide non cosmetic patterns or marks, such as rotation/toric marks, inversion marks, product/brand codes, lot numbers, “DEMO” lenses, and the like, which are of benefits to wearers, eye-care practitioners and manufacturers.
Presently, methods of printing inks onto contact lenses involve cliché ink transfer pad printing. A typical example of this printing follows. An image is etched into metal to form a cliché. The cliché is placed in a printer. Once in the printer, the cliché is inked by either an open inkwell doctoring system or by a closed ink cup sliding across the image. Then, a silicone pad picks up the inked image from the cliché and transfers the image to the contact lens. The silicone pads are made of a material comprising silicone that can vary in elasticity. The properties of the silicone material permit the inks to stick to the pad temporarily and fully release from the pad when it contacts the contact lens.
There are several disadvantages associated with using pad printing to color contact lenses. First, the method lacks consistency and slight differences in the silicone pad can cause wide variation in image quality, effecting dot resolution and color reproducibility. Further, multiple color layering is difficult and time consuming. Further still, the design and printing process using this method is slow. After an image is fully designed, it can take about two weeks before that image is etched onto a cliché. The set-up is painstakingly detailed and lengthy when more than one color is printed on the lens using this method. Lastly, for use in the light stream (glass mold) technology directly contacting the surfaces in selected areas with a silicone pad introduces the potential for oil deposition on the surface and mold release problems. The inconsistency, slow speed of this printing method inhibits business strategies, making it difficult to offer consumers a choice of lens designs at the point of purchase.
Ink-jet printing may be used to replace the above-mentioned method for printing colored images with high quality on a contact lens. However, inkjet inks known in the art are not suitable for printing colored images on a contact lens because they may not meet some basic requirements, such as high opacity without leaching of any harmful residuals, rub-resistance, smoothness, and health safety. Furthermore, inkjet inks are sometimes smeared, streaked, smudged on the contact lens. Many metal oxides are contained in many inkjet inks to provide proper lightness and hue of many inks. The metal oxides are relatively high density and tend to aggregate and form agglomerates that precipitate out of the ink compositions. In addition, iron oxide and titanium dioxide are more difficult to grind to small particles and their suspensions can be less stable. Iron oxides have both electrostatic and magnetic forces contributing to agglomeration while titanium dioxide is a dense, refractive pigment with a hydrophilic surface which is difficult to disperse in a hydrophobic millbase. Consequently, it is difficult to formulate ink compositions containing metal oxide, particularly iron oxide and titanium dioxide that are suitable for use in ink jet printing. It is especially important to be able to incorporate titanium and iron oxides into these inks because these are FDA approved for use in contact lenses and any new, more easily dispersible colorants must go through a lengthy petition process with the agency before they may be used.
Therefore, there are needs for metal oxide dispersion suitable for formulating ink jet inks for printing a high-quality color image on a contact lens without smudging problems using an inkjet printing system. There are also needs for methods for making such metal oxide dispersions.